Why does water from surface water bodies evaporate?

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I mean, isn't the boiling point of water 100 degrees celcius. Even in the hottest of places the temperature does not normally go beyond 45 degrees celcius. So how is all this evaporation taking place?
 

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Andrew Mason
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I mean, isn't the boiling point of water 100 degrees celcius. Even in the hottest of places the temperature does not normally go beyond 45 degrees celcius. So how is all this evaporation taking place?
The molecules of water and air do not all have the same energy. On average they have the same energy but their energies are spread over a range. Air molecules, including free H2O molecules, are all colliding with one another. The same goes for the water molecules. And near the surface, air and water molecules collide. These collisions result sometimes in water molecules in the air sticking to water molecules in the liquid. And sometimes they result in water molecules breaking their hydrogen bonds with others and becoming free.

For any given temperature, there is an equilibrium point at which, on average, the rate at which both occur are the same. This is when the air becomes saturated. So long as the air is not saturated, evaporation will continue to occur.

AM
 
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D H
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Sublimation and evaporation occur when the partial pressure of water is less than the vapor pressure. Boiling occurs when the total pressure is less than the vapor pressure.

It helps to look at a phase diagram.

h2o_phase_diagram_-_color.v2_324.jpg


(Click on the image for a large version.)

Notice that water ice can go directly from the solid phase (ice) to the gas phase (water vapor) if the pressure is low enough. This is called "sublimation." The partial pressure of the water in the atmosphere just has to be below the vapor pressure for sublimation to occur. (Aside: This is why old ice cubes get so rotten.) For temperatures below the triple point temperature, the vapor pressure is the pressure at the boundary of the curve that separates ice and water vapor.

Liquid water evaporates when the partial pressure of water in the atmosphere is below the vapor pressure. For temperatures above the triple point temperature, the the vapor pressure is the pressure at the boundary of the curve that separates water and water vapor.

Boiling occurs when the total pressure is less than the vapor pressure. The difference between water at just under the boiling point and just over it is quite profound. In the first case, there's a good amount of evaporation, but this only occurs at the air/water boundary. In the second case, even the water at the bottom of the pot can't sustain being liquid. Boiling occurs throughout the water while evaporation only occurs at the surface.
 
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Water does not have to be above 100C to evaporate. At the interface between liquid water and air, the partial pressure of the water vapor in the air is going to be equal to the equilibrium vapor pressure of water at the surface temperature of the liquid water. If the partial pressure of water vapor away from the surface is lower than the partial pressure at the surface, there will be a driving force for water molecules to diffuse away from the surface. This constitutes what we call evaporation.

Chet
 

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